Networked communications can use dedicated or switched networks. With a dedicated network, a customer leases circuits that are taken out of public use. This reduces delays and eliminates congestion from outside users, but is costly, hard to configure, and susceptible to catastrophic failure.
By contrast, switched networks generally use publicly shared resources. In circuit switching, such as the publicly switched telephone network (PSTN), once a connection is established it is temporarily maintained on a continuous and exclusive basis. It offers virtually perfect performance but is highly inefficient for anything besides live uncompressed voice or video.
In packet switching, the communication is broken into individually addressed and routed packets. This greatly improves the sharing of network resources compared to a circuit-switched network, thus reducing cost. However, variable packet delays can be introduced by unpredictable variations in route length, hop count, and congestion. Live transmission of voice or video cannot depend on some protocols appropriate for data, such as retransmission of lost packets, as there is not enough time. Nonetheless, the use of packet-switched networks is expanding in response to an increase in data traffic and locations served, and the desire to mix voice and data communications
Asynchronous Transfer Mode (ATM) is a type of packet switching that uses small packets (53 bytes) called “cells.” Because they are small, cells can be formed very quickly (an advantage in real-time communication) and processed quickly with minimal requirements for buffer capacity. ATM is intended to be a single solution for all data types, although it is less efficient for traditional data applications such as large file transfers.
In isochronous (live) communication, ATM gathers voice or video into small packets or cells and presents these to a network in a connection-oriented manner, to deliver quality of service. (This application uses “packets” and “cells” interchangeably.) However, on a shared network, the connection is not absolutely controlled; it may be changed in mid-stream. When a new route is chosen, long gaps or dropped packets can result.
What is needed is a method to ensure smooth communication in the face of rerouted connections in cell-based networks.